GG: I've seen "yfrn" spelled out as "yarn forward round needle," and, yes, it's a yo that is supposed to get you ready for a purl stitch next, but I ignore all those fancy directions (yfrn and yrn, etc. etc.) and just do a regular ol' yo and then go right into my knit or purl stitch.

As far as I can tell, the different abbreviations for a yo really just confuse people more than they help them.